1. Field of the Invention
The present invention relates generally to a lead bonding method for an SMD package, and more particularly to a novel lead bonding method that is capable of properly positioning a lead in the SMD package so as to completely prevent leakage.
2. Description of the Prior Art
As well known to those skilled in the art, a chip embedding package is generally used to mount a device having a specific function, such as a surface acoustic wave(SAW) filter, on a substrate in a rapid and simple manner. A chip embedded in the chip embedding package is electrically connected to a substrate via a lead provided in the package. The chip embedding package is generally classified into a pin-type package the lead of which is projected from it and soldered onto the substrate, or a surface mounted device (SMD) package that can be inserted into a recess formed on a substrate.
Such an SMD package is generally divided into an SMD package using Kovar or a flip type SMD package.
For the SMD package using Kovar, a lead that is electroless-plated with nickel is arranged on Kovar applied on a lead-positioning surface of a package body, and the nickel is seam-welded onto the package body. On the other hand, for the flip type SMD package, a lead and a package body are arranged respectively in a positioning depression or a depression to maintain proper position, and then welded together. A jig or holder has the positioning depression for the positioning of the package body and the lead, and serves as a heater in a final welding step. In general, the jig is made of graphite, and has a stepped structure in the positioning depression to properly position the package body and the lead.
In a lead bonding process of such a flip type SMD package, it is important to properly position the lead and the package body in advance to prevent leakage after the final welding. In order to properly position the lead and the package body, the holder is provided with a number of depression-shaped positioning depressions each having the same shape and size as those of the lead and the package body. In general, the holder utilizes the jig made of graphite.
Hereinafter, detailed description will be given about a conventional SMD package bonding method using a graphite jig 10 with reference to FIGS. 1a to 1d. 
FIGS. 1a to 1d show the steps of the conventional SMD package bonding method. As shown in FIG. 1a, a lead 20 is arranged in a lead-positioning depression surrounded by the stepped portion 10a of a positioning depression 12. The lead 20 includes solder portions 20a of Auxe2x80x94Sn to be bonded to a package body 25. The stepped portion 10a is provided in the bottom of the positioning depression 12 to receive the package body 25 and the lead 20 having lengths different from each other (the lead is 2.35 mm and the package body is 2.5 mm, in general), and defines positioning depressions to fit the lengths, respectively. In other words, the positioning depression 12 provides the lead-positioning depression surrounded by the stepped portion 10a and a package body-positioning depression defined by the sidewall of the positioning depression 12 above the stepped portion 10a. In this way, if the lead 20 and the package body 25 are simply arranged to be fit for the shape of the depressions, the lead 20 and the package body 25 can be automatically positioned properly. Using such a scheme, the lead 20 is arranged in the lead-positioning depression of the positioning depression 12 to be positioned as shown in FIG. 1b, and then the package body 25 is arranged above the lead 20 in the lead-positioning depression to be positioned in the package body-positioning depression defined by the side wall of the positioning depression 12 as shown in FIG. 1c. Then, as shown in FIG. 1c, a current is supplied through electrodes formed at both ends of the graphite jig 10 to heat the graphite jig 10 while the package body 25 is closely pressed against the lead 10 using a pressing means 30 so that the lead 20 and the package body 25 can be bonded together. Here, the graphite jig 10 serves as a heater to melt solder of the lead 20. Through such process, the lead 20 and the package body 25 can be properly positioned to form the SMD package.
However, the deviation of the lead due to malpositioning and resultant leakage generation cannot be completely prevented even in the SMD package bonding method because the positioning of the package body and the lead is dependent upon only proper settlement of the package body and the lead in the positioning depression. Additionally, such improper settlement cannot be detected easily due to the small sizes of the package body and the lead.
FIGS. 2a and 2b show examples of malpositioning.
As depicted in FIG. 2a, when the lead 20 fails to be properly positioned inside of the stepped portion with one side laid on the stepped portion, the package arranged on the lead will not be properly positioned. Therefore, leakage takes place when the package body is arranged on the lead and bonded.
As another example, malpositioning can take place when the package body has a side that is poorly cut in a cutting process.
Referring to FIG. 2b, even if the lead is properly positioned in the lead-positioning depression, the package body with the defective side xe2x80x9cAxe2x80x9d cannot fit into the positioning depression for proper positioning. Therefore, leakage takes place also when the lead is bonded in this state.
To prevent such malpositioning, the position should be inspected separately after each step. However, this has a problem of making the fabrication process of the package complicated.
Moreover, the graphite jig is made of a graphite material that has weak mechanical strength and can be easily damaged or broken. In particular, a damaged portion xe2x80x9cBxe2x80x9d in the stepped portion as shown in FIG. 2c will cause inevitable malpositioning. In other words, since the lead-positioning depression and the package body-positioning depression cannot be formed suitably, desirable positioning cannot be expected for the lead or the package body.
As described above, according to an SMD package bonding method of the prior art, such malpositioning frequently takes place in various forms so that solution is difficult to find. Also, when the stepped structure is damaged, the graphite jig should be replaced with a new one, accordingly increasing production cost.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a lead bonding method, by which a lead and a package body are positioned by a vision system and then fixed through spot welding without using a graphite jig having a stepped structure which can be easily damaged, so that leakage due to malpositioning can be completely prevented.
In order to accomplish the above object, the present invention provides a lead bonding method, comprising the steps of: placing a package body with its lead-positioning surface facing upward; arranging a lead with solder on the lead-positioning surface of the package body; spot-welding the lead onto the package body to fix the lead to the package body; providing the package body spot-welded together with the lead in a positioning depression of a jig with the lead facing downward; and bonding the lead to the package body by melting the solder formed on the lead.
The step of arranging the lead on the package body may include the steps of: photographing the package body; analyzing image data obtained in the photographing step to find position information of the package body; and arranging the lead on the lead-positioning surface of the package body on the basis of the position information found in the analyzing step.
The step of spot-welding may consist of selecting at least one of sides of the lead and spot-welding the lead onto the package body.
The step of spot-welding may consist of selecting central portions of both sides of the lead and spot-welding the lead to the package body.
The step of bonding the lead onto the package body may include the steps of: connecting electrodes at both ends of the graphite jig; and applying voltage to the graphite jig through the connected electrodes to melt solder of the lead.
The step of bonding the lead onto the package body may consist of heating the jig containing the package body and the lead in a calcinating furnace to melt solder of the lead.
The jig used in such bonding process has no stepped structure in the positioning depression unlike the graphite jig of the prior art, and thus has a stable structure with low risk of breakage. Also, advantageously the jig can be manufactured easily.